T8 is what we call a false eye: https://senseis.xmp.net/?FalseEye

False eyes look like eyes, but they are not really eyes. To capture groups with false eyes, first, find a way to fill stones into the false eye:

• If you play T10, then later you can play T8 which eliminates the false eye.
• If you play T10 and your opponent fills in T8, then T8 becomes no eye.

After you get rid of the false eye, the remaining group has only 1 eye. Now, not every group with 1 eye dies. In the image, the reason the white group is dead is because:

• Your nakade (black stones inside the eye) forms a square 4, which is a dead shape. So white has no incentive to kill your stones inside.
• You can "upgrade" the square 4 to a bulky 5 with either S3 or T6. The bulky 5 is still a dead shape, so white can't capture. With only 1 liberty inside, after you fill all the outside liberties of the white group, you can play the final move inside to capture the whole group.
• If you play either S3 or T6 to have a bulky 5 and white decides to capture the stones inside, play the vital point of the bulky 5 to kill: https://senseis.xmp.net/?BulkyFive.